The technique of transient electric birefringence will be used to study the structure of DNA in solution in order to understand the packaging of DNA in vivo and to estimate the strain arising from the mutation causing cyclobutane-type thymine-thymine dimers. The temporal responses of the field free decay and field reversal transient birefringence experiments are very sensitive to the average structure and ionic environment of nucleic acids in solution. Measurements of the rotational diffusion constant of pBR322 restriction fragments as a function of fragment size, ionic strength and temperature will give the lateral stiffness and free energy, enthalpy, and entropy associated with the flexing of DNA. Steric and electrostatic contributions to the persistence length will be separated. The amplitude and dynamics of ionic polarizability of nucleic acids will be elucidated from field reversal experiments on restriction fragments in which the species and concentration of counterions will be varied. The dynamic information will give information about the mobility of counterions in the vicinity of the phosphates in the double helix. The contributions to the polarization from counterions and coions diffusing parallel and perpendicular to the helix axis will be determined. Ultraviolet radiation of DNA converts a portion of adjacent pyrimidine bases into cyclobutane dimers. Local denaturation results from the breaking of hydrogenbonds and the disrupting of base stacking. These dimers have been implicated as biological lesions. The degree of denaturing and the resulting angular deformation of DNA restriction fragments will be measured by studying the rotational diffusion of fragments before and after dimer formation. This technique is a very sensitive way to characterize the small bends whch are probably introduced in the DNA double helix.